The vehicle wheel bearing apparatus that freely rotatably supports a wheel hub that mounts a wheel, via a rolling bearing, includes an inner ring rotation type for a driving wheel and both inner ring rotation and outer ring rotation types for a driven wheel. A double row angular ball bearing is widely used in such a bearing apparatus. This is due to the fact that it has a desirable bearing rigidity, high durability against misalignment and small rotation torque that is required for fuel consumption. The double row angular contact ball bearing has a plurality of balls interposed between a stationary ring and a rotational ring. The balls are contacted with the stationary and rotational rings while applied at a predetermined contact angle.
The vehicle wheel bearing apparatus is broadly classified into a first, second, third or fourth generation structure. The first generation includes a wheel bearing with a double row angular contact ball bearing fit between a knuckle, forming part of a suspension, and a wheel hub. The second generation includes a body mounting flange or a wheel mounting flange directly formed on the outer circumferential surface of an outer member. The third generation includes one of the inner raceway surfaces directly formed on the outer circumferential surface of the wheel hub. The fourth generation includes the inner raceway surfaces formed directly on the outer circumferential surfaces of the wheel hub and the constant velocity universal joint.
In prior art wheel bearing apparatus, since both bearing row arrangements in double row bearing are the same, although it has a sufficient rigidity during straight away running, optimum rigidity cannot always be obtained during curved running. That is, the positional relationship between the wheels and the bearing apparatus is usually designed so that the weight of the vehicle acts substantially in the middle between the rows of bearing balls during straight away running. However, a larger radial load and a larger axial load are applied to the axles of the vehicle on the side opposite to the curving direction (e.g., the left hand side of vehicle when right hand curving). Accordingly, it is effective to have a larger rigidity of the bearing row on the outer side than on the inner side of the bearing row in order to improve the durability and strength of the bearing apparatus. Thus, a known vehicle wheel bearing apparatus is shown in FIG. 16 that can have a high rigidity without enlargement of the bearing apparatus.
The vehicle wheel bearing apparatus 50 is formed by a double row angular ball bearing with an outer member 51 integrally formed with a body mounting flange on its outer circumferential surface. The flange is to be mounted on a knuckle (not shown) of a vehicle. The inner circumferential surface of the outer member 51 includes double row outer raceway surfaces 51a, 51b. An inner member 55 includes a wheel hub 52 with an integrally formed wheel mounting flange 53 at one end to mount a wheel (not shown). One inner raceway surface 52a is formed on the inner member outer circumferential surface opposite to one 51a of the double row outer raceway surfaces 51a, 51b. A cylindrical portion 52b axially extends from the inner raceway surface 52a. An inner ring 54 is fit onto the cylindrical portion 52b. The inner ring 54 is formed with the other inner raceway surface 54a on its outer circumferential surface opposite to the other raceway surface 51b of the double row outer raceway surfaces 51a, 51b. Double row ball groups 56, 57 are freely rollably contained between the outer raceway surfaces 51a, 51b and inner raceway surfaces 52a, 54a of the inner member 55. Cages 58, 59 rollably hold the ball groups 56, 57 in place.
The inner ring 54 is axially immovably secured by a caulked portion 52c. The caulked portion 52c is formed by plastically deforming the cylindrical portion 52b of the wheel hub 52 radially outward. Seals 60, 61 are mounted in annular openings formed between the outer member 51 and the inner member 55 to prevent the leakage of grease contained within the bearing apparatus. Also, the seals 60, 61 prevent the entry of rain water or dusts into the bearing apparatus from the outside.
A pitch circle diameter D1 of the outer side ball group 56 is set larger than a pitch circle diameter D2 of the inner side ball group 57. Accordingly, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger than the inner raceway surface 54a of the inner ring 54. Additionally, the outer raceway surface 51a, of the outer side of the outer member 51, is larger than the outer raceway surface 51b of the inner side of the outer member 51. Also, the number of outer side balls 56 is larger than the number of inner side balls 57. By setting the pitch circle diameter D1 of the outer side larger than the pitch circle diameter D2 of the inner side (D1>D2), it is possible to obtain a large rigidity of the bearing apparatus 50 and thus to extend the life of the bearing (see Japanese Laid-open Patent Publication No. 108449/2004).
However, in such a vehicle wheel bearing apparatus 50 of the prior art, since the outer side end portion of the wheel hub 52 is enlarged, the weight of the bearing apparatus is also increased. Accordingly, although the prior art bearing apparatus can increase rigidity, it has an adverse effect of increasing its weight.
Also, in the prior art bearing apparatus 50, a stepped portion 63 is formed on the wheel hub 52 between the inner raceway surface 52a of the outer side and the cylindrical portion 52b, which includes the inner ring 54. The presence of the stepped portion 63 (height of step: (D1−D2)/2) causes a ball problem. The balls 56 of the outer side temporary assembled in the outer raceway surface 51a of the outer member 51 tend to contact the counter portion 62 of the inner raceway surface 52a. Accordingly, the stepped portion 63 of the wheel hub 52 may damage the balls 56 during assembly of the bearing apparatus 50.
In addition, a problem may occur in that micro damages may be caused on the surfaces of the ball groups 56, 57 during temporary assembly of them while they are held by the cages 58, 59 into the double row outer raceway surfaces 51a, 51b of the outer member 51. That is, micro damage (ball damage) is often caused since the balls 56, 57 are inserted into the inner circumferential surfaces of the counter portions 64, 65 of the outer raceway surfaces 51a, 51b while being abraded by corners or irregular turned surfaces of the counter portions 64, 65. The damaged surfaces of the balls 56, 57 cause noise to the bearing apparatus and reduce the life of the bearing apparatus. Accordingly, very careful assembling work is required. This reduces assembling efficiency of the bearing apparatus.